The pattern of activity in the circuits of the brain and their experience-dependent changes underlie the processing of sensory information, perception, and memory. Much has been learned about the anatomical wiring of brain circuits and about the receptive field properties of individual neurons in the intact brain, but considerable mystery remains about how the properties of individual neurons emerge from their connectivity and how their activity is assembled into coherent percepts. Part of the problem has been that high precision recording is usually obtained from only one or a few neurons at a time, when salient events are actually processed by large assemblies of neurons. The method with the best resolution in time and space currently for population recording is the optical recording of neural activity with voltage-sensitive chemical dyes. However, these optical reports are blurred by their lack of targeting to specific cell types or subcellular compartments. Glial signals are confounded with ones from neurons, distinct types of neurons contribute indistinguishably to the signal, and the neuronal component of the signal is dominated by the dendrites, which represent the major membrane area. What has been missing is a method of targeting these optical reporters. The goal of this grant is to create genetically-encoded optical reporters of neural activity to fill this gap. The advantage of this approach is that the reporters can be specifically expressed in desired cell types through the use of cell-type-specific promoters, and they can be targeted to specific subcellular compartments. Our emphasis is to develop reporters that reveal action potential firing, synaptic activity, second messenger signaling, and changes in the molecular assembly of presynaptic active zones and postsynaptic densities. These reporters will be expressed in cultured cells to optimize their properties and then delivered transgenically and via viruses to model systems (the Drosophila larval neuro-muscular junction, and the mouse and rat barrel cortex) in which they will be tested in vitro and in vivo. [unreadable] [unreadable]